The well-known Hall-Heroult process produces aluminum by electrolysis of alumina dissolved in a molten fluoride salt bath maintained at temperatures of 900–1000° C. Alumina (Al2O3) produces aluminum and oxygen when it breaks down. Aluminum is collected in a molten layer below the anode and oxygen is released adjacent the anode.
Carbon is used as the anode material because oxidation-resistant anodes are not yet commercially available. Carbon is consumed in relatively large quantities in the process, generally about 420 to 550 kg. carbon per metric ton of aluminum produced.
A new anode includes a carbon block joined by stubs and an iron yoke to an aluminum or copper anode rod. The height of the carbon block in a new anode is about 62 cm. Its life span in a cell is about 27 days after which the height of the carbon block is reduced to about 15 cm. The spent anode must be replaced before it is completely consuned in order to avoid the risk of contaminating aluminum with steel from the stubs or with cast iron used for joining stubs into the carbon block. A small aluminum plant having 264 cells may replace close to 400 anodes per day, requiring about 150,000 anode replacements per year.
When a new anode replaces a spent anode in a cell, its height must be positioned accurately in order to assure efficient operation of the cell. The new anode should also be positioned quickly in order to minimize gas emission and cell perturbations. Several processes and apparatus for replacing anodes have been developed in the prior art. Some prior art patents covering various aspects of anode changing include Messina U.S. Pat. No. 3,850,305; Kato et al. U.S. Pat. No. 4,032,020; Duclaux U.S. Pat. No. 4,465,578; Skaar et al. U.S. Pat. No. 4,992,146; Marttila et al. U.S. Pat. No. 5,151006; Luebke et al. U.S. Pat. No. 5,730,855; and Zannini U.S. Pat. No. 5,435,897. However, there still remains a need for an efficient and economical process and apparatus for positioning new anodes accurately and quickly in an aluminum electrolysis cell.
A principal objective of the present invention is to provide an efficient and economical process and apparatus for automatically positioning the height of new anodes in an aluminum electrolysis cell.
A related objective of the invention is to provide a process and apparatus for reducing variations in the height of new anodes among different individuals operating the electrolysis cell.
An advantage of the present invention is that vertical positioning of new anodes is minimally subject to variations in position of the overhead crane supporting the anode changing apparatus.
Additional objectives and advantages of the invention will become readily apparent to persons skilled in the art from the following detailed description of some particularly preferred embodiments.